Why do people's dna patterns differ so greatly?

Picture in your mind two people you know really well. How much are they really alike, sure two arms, two hands, two feet, two leg, and so forth. How about color of eyes, hair, skin or texture of skin, fineness or coarseness of hair, shape of nose, skull, overall body shapes and so many other things we cannot examine with our eyes. 

DNA are parallel ribbons containing the gene patterns that make us unique individuals. There can be almost endless combinations as suggested above with just those few. Our genetic makeup is programmed to produce each little facet of our body. No single part of us is like any one else, just similar. There are about four blood types, maybe five, hair colors, and endless shapes. That is why we are so precious, or should be considered precious because we are unique and that is awesome. 

Now when you trace a family or an isolated population DNA, you begin to see fewer difference. That is because without the introduction of new genes the same pool of genes can only be mix so many ways. The smaller the gene pool, the fewer mixes. Then there are unique transmissions of genes that can be identified that is passed only through females. Therefore if a mother had a daughter, the daughter would have a tracing pattern that showed that and that can occur accurately for many, many generations. Interestingly enough, it is suggested that every female can be traced to the first female, Eve of the Garden of Eden. That is awesome. 

Hope this helps.